EAF steelmaking is a highly energy-intensive process. It consumes a large amount of electrical and chemical energy. The hot heel is the liquid steel remaining in the furnace after the preceding melting operation. Such a hot heel is remaining in many traditional steel melting processes due to practical aspects. When a steel melting process is completed the melted steel traditionally was discharged simply by tilting the furnace, whereby a part of the melted steel due to the geometry of the furnace was left. A further reason for maintaining a hot heel has been to avoid too large temperature fluctuations in the walls of the furnace. A positive effect of the hot heel is further that electric energy supplied for the melting process can be reduced.
In order to provide optimal conditions for the process it is advantageous to provide stirring of the melt. This is because without stirring, the melt is so stagnant that it leads to severe problems. Cold spots due to non-uniform melting occur. In order to reach a desired steel quality, the melting time and bath temperature have to be increased to reach a flat bath. This results in high energy consumption. Further, it takes long time for large pieces of scrap to be melted. Severe thermal stratification of the steel melt with high bath temperature gradient will occur. The gradients cause uneven reaction area.
By stirring such related problems can be eliminated or at least reduced. Known main technologies applied for stirring are electromagnetic stirring (EMS) and gas stirring. Gas stirring has some disadvantages in comparison with EMS.
EMS can move solid scrap transversely, which is not the case for gas stirring. EMS has better stirring effect for scrap melting. The stirring effect of gas stirring is also limited due to insufficiency of fitting EAF geometry. Gas stirring has low reliability due to maintenance and operation difficulties and entails the risk for bottom inject hole blocking. Leakage of molten steel from gas injection holes might occur.
For an effective EMS it is necessary to have a substantial part of the furnace content to be in liquid state. With a hot heel of a size traditionally applied, the charging of scrap into the hot heel results in that at least part of the hot heel solidifies since it is cooled down by the scrap. Then the proportion of the liquid state will gradually increase due to the heating by the electric arc and it will take some time until the liquid fraction is sufficiently large to effectively start the EMS. This is normally not present until a large part of the process has passed, typically about 40-50% of the total process time. With process time is normally meant the time span from the initial charging of the furnace with the metal scrap until the molten steel is discharged therefrom, and this is the meaning also in the present application.
The benefits from applying EMS therefore are gained only during a part of the process.
An illustrative example of prior art is disclosed in GB 2192446 A describing a steel melting process applying gas stirring. The furnace operates with a hot heel of a limited size that is mentioned in the disclosure to be in the range of 10 to 30% of the previous charge. As mentioned above, the mass of such a limited hot heel is normally not sufficient to avoid solidification of parts of the hot heel when the scrap is charged. Stirring at an early stage of the process therefore will not be effective, in particular not when using EMS.
DE 9422137 U1 also discloses a melting process in which a hot heel is maintained from the previous process cycle. The mass of the hot heel is not defined and if the hot heel has a mass within the range conventionally present it would not be sufficient to avoid solidification of at least parts of the hot heel.
EMS stirring as such is disclosed e.g. in GB 1601490.